unknown quantity
An unpredictable person or thing, as in We don't know how the new pitcher will do—he's an unknown quantity. This expression comes from algebra, where it signifies an unknown numerical value. Its figurative use dates from the mid-1800s.
Words nearby unknown quantity
unknit,
unknot,
unknowable,
unknowing,
unknown,
unknown quantity,
unknown soldier,
unlace,
unlade,
unlaid,
unlamented